1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, to an electrode of an electrode assembly of a secondary battery.
2. Description of the Related Art
According to usage and battery capacity, secondary batteries are classified into low-capacity secondary batteries, which are referred to as “small batteries” hereinafter, that use a single battery cell packaged in the form of a pack, and high-capacity secondary batteries, which are referred to as “large batteries” hereinafter, that use scores of battery cells packaged into a battery pack for driving a motor.
Small batteries are used as the power source for small electronic devices, such as mobile phones, laptop computers, and camcorders, while large batteries are used as the power source for driving motors in hybrid electric vehicles and the like.
Depending on the external shape, small batteries may be classified into different types, such as square and cylindrical batteries. A small battery generally includes a positive electrode, a negative electrode, and a separator that is interposed as an insulator between the positive electrode and the negative electrode, that are spiral-wound to thereby form an electrode assembly. The electrode assembly is inserted into a cylindrical case to form the battery.
Each of the positive and negative electrodes of a secondary battery is provided with a conductive lead element for collecting current generated in positive and negative electrodes during the operation of the secondary battery. The lead element induces current generated in the positive and negative electrode to positive and negative terminals.
When the structure of the small battery is directly applied to a large battery, the operational characteristics of the large battery are not fulfilled with respect to electric capacity and power output. Therefore, a multi-tab structure using a plurality of tabs which are attached to an electrode assembly has been suggested in Japanese Laid-open Patent Application No. 2003-7346. This battery has a plurality of tabs along one direction of electrode assembly and the tabs are combined with an internal terminal which is connected to an external terminal.
However, such a multi-tab structure is required for many working processes. Especially, the tab has a small unit area and thus limitations for satisfying the output power characteristics required for large batteries.
Another form of the lead element is a current collecting plate. Since the current collecting plate has a wider unit area than the tap, it can have an increased current collecting efficiency and a higher energy density per unit area than the tap by reducing the space occupied by the tap in the case.
The current collecting plate, however, has a shortcoming that it is not fixed on an electrode assembly firmly. To be specific, the current collecting plate is usually affixed to the electrode assembly mainly by welding. If the electrode assembly is formed in a shape of a jelly-roll by winding a positive electrode, a negative electrode and a separator, and a current collecting plate is fixed on the electrode assembly by welding, the contact area between the current collecting plate and the electrode assembly is relatively small in the outer part, compared to the contact area in the central part of the electrode assembly. Therefore, the welding is not carried out properly in the outer part and this makes the connection between the current collecting plate and the electrode assembly unstable.